1. Field of the Invention
The invention relates to an apparatus for the production of grains according to the rolling layer technique.
2. Description of Related Art
In various technical fields, e.g. in the chemical, pharmaceutical, and food industries as well as in agriculture, the production of spherical or approximately spherical grains of 1-10 mm size is often a necessity. The effort directed to the production of material available in this form, --in addition to the special advantages in the existing particular field and the aesthetic appearance of the product--has the following important general reasons: good abrasive resistance; high breaking strength; adjustable and easy feeding (smooth flow); homogeneous (quasi-homogeneous) space utilization; minimal grain surface and volume ratio, and beneficial hydrodynamic characteristics. One of the possible methods for producing sets of grains having such properties is the rolling layer technique known in several versions.
Rolling layer is understood as a special motion-form of a set of grains which facilitates execution of physical, physiochemical and chemical processes, in the course of which in the interior of an axially symmetrical apparatus-body--as a result of the total effect of friction, gravitation and centrifugal forces--the whole set, and within it, each grain performs a characteristic rolling-rotary motion. This motion basically depends on the speed (r.p.m.) of the apparatus body. The mentioned physical process may be for example grinding, granulation, application of a coating layer; physiochemical process is for example the calcination and chemical process is the ammoniation.
The devices--apparatuses--of the rolling layer technique are known in several forms and sizes; these apparatuses are mostly called "rotary vessel", "drum" or "plate" (P.J. Sherrington, R. Oliver: "Granulation", Heyden and Son Ltd. 1981; K. Meyer: "Pelletizing of Iron Ores", Springer Verlag, Berlin, 1980).
If the work involves size-increasing and forming sets of grains or powders, then basically the following processes take place in the rolling layer: nucleation (agglomeration of primary grains); fusion (coalescence of agglomerates), and surface layering (sticking of primary grains on the surface of nuclei or larger agglomerates) (C.E. Capes: "Particle size enlargement", Elsevier Scientific Publishing Co., 1980).
It is evident, that if the enumerated basic processes (furthermore the additional stochastic processes not detailed here) could take place jointly in time and/or space, then the derived product--regarding the grain size distribution--would be heterogenerous and the individual grains would not be spherical by necessity. On the other hand, the condition of producing virtually spherical grains of nearly the same size is that the grains should be growing mainly according to the mechanism of ideal layering, i.e. at the same rate on all points of the grains--and the agglomeration should take place separately in space and/or time, at an optionally controlled rate, mainly limited to the possible necessary nucleation. But the possibility of agglomeration, e.g. coalescence of the grains close to the required size of the product should be definitely precluded.
With the apparatuses based on the currently known rolling layer technique, the mentioned conditions cannot be fully satisfied, consequently a product of suitable quality--virtually regular spherical sets of grains having 1-10 mm grain size, powder-free surface, good mechanical characteristics, and regarding the average grain size, a relative grain distribution of a maximum 0.2--cannot be ensured in continuous and stationary operation. A product with such parameters can be produced only in an intermittent operating mode, so that the formation takes place mostly with a so-called size enlarging-forming coating.
The essence of this method is to bring the grains--nuclei--smaller than the required size to a rolling motion in a rotary vessel or drum, then the surfaces are uniformly wetted with automization of liquid until the limit of adhesion. This is followed by applying uniformly coating powder to the rolling layer--e.g. through a vibration sieve--until the powder is set on the surface of the grains. The wetting and powdering are repeated until the required size and shape are reached. If necessary, the wet grains are compacted by after-rolling. The nuclei can be formed in a separate step from the powder with a granulation process, but occasionally foreign nuclei are also used; e.g. when coating vegetable seeds, the nucleus is already given.
Catalysers and catalyser-carriers can also be formed with the method similar to the one used for coating of vegetable seeds. Such solutions were disclosed in the European patent specification No. 75,314 and U.S. Pat. No. 4,255,253. Similarly, the formation of adsorbents can also be accomplished.
The common drawback of all the presently known intermittent grain-forming processes based on size-enlargement coating is the low productivity and very high labour demand.